This invention relates to electron-beam machines as used in the fabrication of semiconductor circuits. Basically, in such machines, electrons are emitted from a cathode, accelerated by an electric field between the cathode and an anode, and directed by several lenses along a path to form an image of predetermined shape at a target. In the semiconductor industry, the image generally is a round or rectangular or square shaped spot, and the target is a layer of electron sensitive resist on a semiconductor substrate.
Typically, the spot is less than one micron across. The spot is moved to bombard only selected portions of the resist on the target. Those portions of a positive resist which are bombarded have their chemical properties changed such that they can thereafter be chemically removed from the substrate. But the unbombarded portions of a positive resist are insoluble in the chemical and thus remain as a mask for further processing of the substrate.
One electron-beam lithography system which is typical of those used in the prior art is illustrated in FIG. 1. In that figure, reference numeral 10 indicates the cathode. Electrons from the cathode are accelerated by an anode (not shown) through the aperture 11 of an iris 12. Reference numerals 13a, 13b, 14a, 14b, 15a, and 15b indicate various paths by which some of the electrons from cathode 10 pass through aperture 11.
In order to form a demagnified image of aperture 11, the system of FIG. 1 includes a lens 16. For illustrative purposes, lens 16 as well as all other lenses in this case, are drawn as optical lenses; but they actually are electro-magnetic lenses. Electrons on paths 14a and 14b pass through the center of lens 16, and thus they emerge from that lens along undeflected paths 17a and 17b. But electrons along paths 13a and 13b are deflected due to the operation of lens 16 along paths 18a and 18b, respectively.
Reference numeral 19 indicates the demagnified image of aperture 11 which is formed by lens 16. Its location is determined by the intersection of paths 17a and 18a, and the intersection of paths 17b and 18b. By inspection of those paths, it can be seen that the amount by which image 19 is demagnified is dependent upon the angle by which lens 16 bends paths 18a and 18b. A greater bend results in more demagnification.
Unfortunately, due to various technological limitations, the most demagnification which is realistically feasible is approximately 1/10th. Stronger lenses are not feasible because they are limited by magnetic saturation or overheating due to the large number of ampere-turns which would be required. At the same time however, aperture 11 must generally be about 300 micrometers across. Any smaller aperture will reduce the number of electrons that pass through it which reduces current at the target below an acceptable level.
Thus, to obtain an image of aperture 11 which is less than one micrometer across, additional lenses must be provided. These are indicated by reference numerals 20 and 21. Lens 20 forms a reduced image 22 of image 19; and lens 21 forms a reduced image 23 of image 22. Reference numerals 24a-26b indicate some of the electron paths from image 19 which form image 22; and reference numerals 27a-29b indicate some of the electron paths from image 22 which form image 23. With this system, image 23 can typically be made to be one-half micron across.
However, an undesirable aspect of the FIG. 1 electron-beam system is that it requires too many lenses for its operation. Each lens adds to the cost of the system. Also, the entire path over which electrons pass through the lenses must be encapsulated in a vacuum column; and the cost of the system increases with the length of that column.
Another problem with the FIG. 1 system is that the percentage of electrons from cathode 10 which actually hit the target is too low. More than 50% of the electrons which are emitted by cathode 10 are blocked by plates 30, 31, and 32 which respectively surround lenses 16, 20, and 21. See for example, the electrons which pass along paths 15a, 15b, 26a, 26b, 29a, and 29b. Electrons on those paths could only be captured by increasing the size of each lens; but that in turn would reduce the lenses demagnifying power and consequently even more lenses would have to be added to the system.
Still other problems occur in the FIG. 1 system due to the crowding of electrons which occurs at the point where various paths cross over one another. This crowding of electrons occurs at the point where paths 26a and 26b cross, at the point where paths 29a and 29b cross, and at the cross over point just before the formation of image 23. Due to this crowding, the electrons are repelled from each other by coulomb forces. The forces occur in both the horizontal direction and vertical directions, and the effect is to make the image 23 at the target less sharp.
Further, in the electron-beam system of FIG. 1, cathode 10 should be at least as large as aperture 11. This insures that some electrons will travel along paths 13a, 13b, 14a, and 14b and thus form image 19 of aperture 11. However, only thermionic cathodes can be made at least as large as aperture 11, which typically is 300 micrometers across. But a problem with thermionic cathodes is that their emission is relatively low (i.e., 10-25 amps per centimeter squared), and their operating lifetime is also relatively low (i.e., 100 hours) at these current densities.
Accordingly, a primary object of the invention is to provide an improved electron-beam lithography system.
Another object of the invention is to provide an electron-beam system having a reduced number of components.
Another object of the invention is to provide an electron-beam system having a high percentage of emitted electrons which actually reach the target.
Another object of the invention is to provide an electron-beam system wherein distortion at the target caused by electron crowding in their path to the target is reduced.
Still another object of the invention is to provide an electron-beam system having increased current density at the target and increased lifetime.